The present invention relates to evaluation of properties of fluids used in mechanical systems, such as lubricating oil and hydraulic fluid, including water content, viscosity, acid content and density of the fluid.
Mineral and synthetic working fluids, such as motor oil, gear oil and hydraulic liquids, are frequently used essential components of mechanical systems. Working fluids provide lubrication and/or force and energy transfer in the mechanical system. Unfortunately, working fluids are subject to degradation with use over time. For example, working fluids may be contaminated by water or debris. In addition, contamination of working fluids by water and dissolved air leads to accumulation of oxide products, increasing the fluid viscosity and reducing its lubricating effect. Such contamination and degradation leads to increased friction in the mechanical system and ultimately to premature failure due to wear.
In many industrial environments, working fluids are regularly analyzed to determine if fluid breakdown is occurring, threatening mechanical failure. Laboratory methods for detecting working fluid viscosity and degradation are described in ASTM standards D445-94 entitled Kinematic Viscosity of Transparent and Opaque Liquids (the Calculation of Dynamic Viscosity), D95-83 (1990) entitled Water in Petroleum Products and Bituminous Materials by Distillation and D664-95 entitled Acid Number of Petroleum Products by Potentiometric Titration.
Unfortunately, accurate oil analysis typically requires shipment of a fluid sample to a laboratory for analysis. This results in latency between the sampling of the working fluid and the generation of analysis results. This problem is particularly vexing where the mechanical system is in a moving vehicle such as a ship or aircraft, or is located in a rural area such as a factory.
Unfortunately, these laboratory methods have major weaknesses. First, laboratory analysis has a significant cost, and typically cannot be performed on-site. Furthermore, if a working fluid sample must be transported off-site for analysis, a significant delay will occur between sampling and receipt of the test results. A relatively simple method for working fluid analysis which does not require extensive laboratory equipment and can be performed on-site without expertise, would clearly be preferable. Ideally, the simplicity of the method would permit an analyzing device for evaluating the working fluid, to be incorporated into the mechanical system, so that analysis of oil in the system is performed continuously while the system is on-line, i.e., still operating. Such an approach would eliminate all unnecessary downtime, because the system could be operated continuously until the oil is found to need replacement, and the replacement can be made immediately rather than after an oil sample has been delivered to a laboratory and analyzed. Furthermore, in automotive applications, an on-line oil analysis method that identifies when oil should be changed, could in many instances lead to greater mileage between oil changes reducing maintenance costs, environmental strain caused by disposal of waste oil, and inconvenience.
Several proposals have been made for performing on-site or on-line analysis of working fluid using relatively simple analyzing devices. One popular proposal involves measuring dielectric properties of the working fluid and implying properties of the working fluid from the results. For example, U.S. Pat. No. 4,646,070 describes a sensor including a pair of electrodes which are placed in an oil-carrying passage of the mechanical system, and immersed in the flowing oil to form a capacitor. The capacitance of this capacitor varies as a function of the permittivity of the oil. A measuring circuit determines the permittivity of the oil and generates a measure of the quality of the oil. U.S.
Pat. No. 4,733,556 describes a sensor with two pairs of capacitive electrodes. The first pair of electrodes is immersed in oil flowing in the operating mechanical system, and a second pair of electrodes is immersed in reference oil. A circuit compares the capacitance measured from the two pairs of electrodes, and produces a measure of the quality of the oil. Other sensors measure the variation of the permittivity of oil over frequency, and use the resulting data to determine a measure of the quality of the oil.
Unfortunately, these proposed systems have not been widely accepted for measurement of oil parameters. The permittivity of a working fluid such as oil depends on a large number of independent parameters. For example, the concentration of oxidized molecules (acid content), water content, particulate content, viscosity and temperature of oil all influence the permittivity of oil and its variation over frequency. Thus, a measurement of permittivity per se or the variation of the permittivity over frequency will not determine a value for any one of these parameters independent of the others. However, to adequately characterize the performance of a working fluid and determine whether the working fluid should be replaced, all of the parameters of water content, acid content, viscosity and density, need to be independently and accurately measured, and then evaluated separately. The quality measurements produced by the systems described in the above patents, are dependent upon several different oil parameters, and thus cannot be related to any one parameter or easily used to answer the basic question of whether oil should be replaced.
Soviet patent 1,566,291, authored by an inventor of this application, describes a method for analyzing the properties of oil based on variation of dielectric parameters of the oil over temperature. This method produces measurements of parameters of oil that are independent of most other parameters. In this method, a capacitive-type sensor is immersed in the oil, and a circuit stimulates the sensor to determine the dielectric loss in the capacitive sensor. The dielectric loss of the oil (which results from the imaginary part of the complex function for permittivity) varies non-linearly with temperature. In accordance with the method of the Soviet ""291 patent, the oil is heated while monitoring the change in the dielectric loss. The ratio of the imaginary to the real part of the permittivity is known as tangent delta or tgxcex4. At the oil""s xe2x80x9ccritical temperaturexe2x80x9d, tgxcex4 reaches a maximum value. This critical temperature is identified by monitoring tgxcex4 as the oil temperature is increased, and identifying the temperature at which tgxcex4 ceases increasing and begins decreasing. The determined value of the critical temperature is converted to a measure of the viscosity of the oil, using a calibrated plot shown in FIG. 4. The determined maximum value of tgxcex4 is converted into a measure of the acid content of the oil using a calibrated plot shown in FIG. 5.
Unfortunately, the method described in the Soviet ""291 patent is flawed in several ways. First, the described method for locating the critical temperature is not accurate. Measurement variations can produce an apparent decrease in tgxcex4, incorrectly suggesting that the critical temperature has been located, resulting in a mis-determination of the critical temperature and mis-determination of the oil viscosity. Furthermore, the value of tgxcex4 at the critical temperature is not only related to the acid content of the oil, it is also related to the water content of the oil. Accordingly, in the described method, variation in water content of the oil can lead to an incorrect measurement of acid content.
Accordingly, there remains a need for a method for measuring parameters of oil including viscosity, density, acid content and water content, independently of other parameters, which is accurate and requires relatively simple analyzing devices suitable for on-site or on-line applications.
The present invention provides a method and apparatus for analyzing working fluid which meets these objectives. In the embodiment described below, the permittivity of the working fluid is measured as the temperature of the working fluid is varied over a range, which range includes a special temperature where the rate of change of the permittivity over temperature is at a maximum. Subsequently, various parameters of the working fluid can be determined.
In one aspect of the present invention, the value of the special temperature is directly converted to a measure of viscosity of the working fluid, using an appropriately generated plot of viscosity relative to special temperature.
In a second aspect of the present invention, the rate of change of the permittivity with respect to temperature is directly converted to a measure of the acid content of the working fluid, using an appropriately generated plot of acid number relative to the rate of change of the permittivity at the special temperature.
In a third aspect of the present invention, the value of the permittivity of the working fluid at the special temperature is directly converted to a measure of the moisture content of the working fluid, using an appropriately generated plot of moisture content relative to the value of the permittivity at the special temperature.
In a fourth aspect of the present invention, the rate of change of the permittivity of the working fluid with respect to temperature, below the special temperature, is directly converted to a measure of the density of the working fluid, using an expression relating working fluid density relative to the rate of change of the permittivity below the special temperature.
In a further aspect, a working fluid attribute is determined accurately by a curve-fitting technique. Measurements of a working fluid parameter (in the above aspects, this parameter is the fluid""s permittivity) are taken over a variation in an environmental variable (in the above aspects, this variable is temperature). The measurements are then fitted to a mathematical model of the expected curve of the variation in the working fluid parameter over variation in the environmental variable. Finally, this mathematical model is analyzed to identify an attribute of the working fluid.
In the embodiment described below, this curve-fitting technique is used to form a mathematical model of the curve of the variation in the permittivity over temperature. Then, the special temperature is identified by locating the temperature at which there is an inflection point in the model curve, and this temperature is used to identify the viscosity of the working fluid. Further, the mathematical model is used to compute the rate of change of the permittivity with respect to temperature, and this rate of change is used to identify the acid content of the working fluid. Also, the mathematical model is used to compute the value of the permittivity of the working fluid at the special temperature, and this value is used to identify the moisture content of the working fluid. Finally, the mathematical model is used to identify the rate of change of the permittivity of the working fluid with respect to temperature, below the special temperature, and this value is used to identify the working fluid density.
In each of the above embodiments, the working fluid may be oil, hydraulic fluid, or any other fluid used in a mechanical system which is subject to contamination and degradation. Furthermore, principles of the present invention could also be applied to analysis of other hydrocarbon liquids or other working fluids/chemicals having a dipole moment and exhibiting dielectric relaxation over change in an environmental variable such as temperature or applied frequency of electrical excitation.
Other aspects of the invention are also disclosed. For example, stated generally, the invention relates to a method for analyzing a working fluid to determine an attribute of the working fluid, by generating a dielectric relaxation spectrum of the permittivity of the working fluid. The dielectric spectrum is produced by varying an enviornmental variable affecting the permittivity of the working fluid (which may be temperature, or frequency) over a range including a special point at which the rate of change of the permittivity with the environmental variable is at a maximum. Then, the special point in the dielectric relaxation spectrum is identified from variation in the permittivity of the working fluid, and then features of the dielectric relaxation spectrum at the special point to a measure of the attribute of the working fluid.
In the specific embodiment disclosed below, the dielectric spectrum is generated by varying the temperature of the working fluid rather than the frequency of the applied electrical stimulation, for the reason that variation of either temperature or frequency will generate a dielectric spectrum, but the applied frequency can more easily be controlled to a constant value while varying temperature, than vice versa.
Additional aspects of the invention feature apparatus for performing the above analysis method. This apparatus may be in the form of a desk-top oil analyzer for use in an on-site application, or an integrated analysis device suitable for mounting directly to the mechanical system, e.g., in the form of an adapter insertable between the oil filter and oil filter mounting.